This invention relates to a domestic satellite communication system (DSC system) and, more particularly, to a DSC system for voice communication between subscribers served by subscriber earth stations via one single satellite link.
Generally, satellite communication systems are suitable for long-distance communication since construction cost and transmission quality are less relevant to the distance compared costs for land communication systems. They are especially effective and less costly when constructed as a domestic communication system in countries having a vast area, geographically dispersed islands, or still undeveloped. In the domestic satellite communication systems between subscribers served by subscriber earth stations is carried out via a satellite on a stationary orbit at an altitude of approximately 36,000 km. It inevitably causes a propagation delay which is incomparably longer than delays in an overland communication system.
The delay time generally extends for about 260 ms per satellite link (i.e., from an earth station to a satellite and return an earth station). Most DSC systems uses a single central earth station which concentratedly controls demand assignments for satellite channels when a demand arises so that satellite channels with a limited number of radio frequencies are flexibly assigned to suit varying traffic volume. When voice communication is conducted between subscribers of subscriber earth stations which are controlled by such a central earth station via a demand-assigned channel of a satellite, it becomes indispensable to pass the communication through the channel connected to a central switching system, as well as to charge for such subscriber-to-subscriber communication through the central switching system. In such a case, however, because two satellite links exist between the subscriber earth stations, channels cannot be used at a high efficiency. Moreover, since voice signals experience the propagation delay of approximately 520 ms in two such links, voice communication becomes difficult.
U.S. Pat. No. 4,273,962, discloses an example of a DSC system which can achieve an efficient use of satellite channels and minimize a propagation delay of voice signals. However, this system cannot prevent an over-charge for communication between the calling subscriber and the called subscriber, via one satellite link, if a fault occurs in a satellite channel during the continuity of a call because a charge (an impulse metering)) in the calling subscriber earth station continues until either the calling subscriber hangs up or the calling subscriber earth station hangs upon command from the central earth station (the master station).